Conventionally, the general relationship between an AC adaptor of this type and an electronic apparatus is such that the AC adaptor is given constant-voltage/constant-current characteristics suited to battery charge, and, when the electronic apparatus is to be driven, constant-current control is temporarily delayed to control a rush current generated in the electronic apparatus.
Also, as disclosed in Japanese Patent Laid-Open No. 8-223907, a built-in charge controller which is incorporated into an electronic apparatus and performs constant-voltage/constant-current control is proposed.
Unfortunately, in a power supply apparatus of this type, it is difficult to control the maximum value of a temporal large electric current as measure to counter a rush current. Since a large-capacity element for allowing a large electric current is necessary, the system is disadvantageous in cost.
In addition, an electric current corresponding to the rush current is temporarily supplied to a battery during charging. Consequently, excess stress is applied to the battery and its internal protective element. Also, if the period during which the constant-current control is temporarily delayed is shortened, the above limitation adversely affects the operation of the electronic apparatus in some cases.
Furthermore, in Japanese Patent Laid-Open No. 8-223907 described above, the voltage of a region in which the rush current is generated in the electronic apparatus is set higher than that of the battery charging region. Accordingly, a large-capacity element which permits high power is necessary in respect of heat design in order to hold the maximum DC output. That is, this prior art has no effect on downsizing and cost reduction of the AC adaptor at all.